First, none of the original gospel manuscripts mentions their author. Second, dating John in the 2nd century used to be fairly standard, but today the consensus has shifted much closer to 70 AD. While there are more than one hand at work, the central author certainly had a fine grasp of the context of Jesus' ministry. He was familiar with people like Pilate and Caiaphas. Third, the gospel's theology is highly sophisticated. Richard Bauckham's thesis is that "John" was a young man who followed Jesus, but spent decades mulling over what he knew before writing the bulk of the text.